I was on World Campus Afloat in 1973 when I and a couple of people noticed a dark shadow with no sound moving through the water. IT was NOT a shadow from above but UNDER the water. It moved at high speed around the harbor of the Seychelle Islands where the SS Universe was docking. The object appeared to be 10 to 15 feet long and did not disturb the water. However it was moving at speeds too fast for any water animal, fish. There was NOTHING in the air at the time to cast a shadow, and the shadow appeared to be BELOW the surface. Despite 42 some odd years i still remember this quite well. I had sort of put it out of my mind but recent descriptions of Under Water UFO's on History jogged my memory.